1. Field of the Invention
The present invention relates to imaging lenses to be built in image pickup devices using a solid-state image sensor such as a relatively compact and thin CCD sensor or CMOS sensor which are mounted in mobile terminals or PDAs (Personal Digital Assistants), such as mobile phones and smart phones.
2. Description of the Related Art
Today, mobile terminals and PDAs, such as mobile phones and smart phones, usually have a camera function which provides high resolution to cope with an increase in the number of pixels. In addition, for the purposes of increased convenience and better appearance of mobile terminals and PDAs, the demand for smaller and thinner image pickup devices which are mounted therein is growing. Therefore, an imaging lens which is built in an image pickup device is also strongly expected to provide higher resolution, more smallness (compactness) and more thinness (low-profile design) and also be a fast lens (namely a lens with a small F-value) and at the same time, to provide a wide angle of view to capture an image of an object in a wider perspective.
In order to meet the recent trend toward an increase in the number of pixels, as an imaging lens built in an image pickup device as mentioned above, an imaging lens composed of five lenses has been proposed which can provide higher resolution and higher performance than an imaging lens composed of four lenses. However, although in order to cope with an increase in the number of pixels it is effective to increase the number of constituent lenses, this approach makes it difficult to realize an imaging lens which meets the need for more compactness and thinness.
In order to correct chromatic aberration, many of the existing imaging lenses composed of five lenses use a combination of positive lenses made of a low-dispersion material and negative lenses made of a high-dispersion material. However, when the lens configuration includes many lenses with negative refractive power, the total track length tends to be long. In an attempt to shorten the total track length in this type of lens configuration, it is necessary to increase the positive lens refractive power. However, if the curvature radius of a lens surface is decreased to obtain the required positive refractive power, manufacturing error sensitivity would increase, leading to a lower productivity. In addition, increased positive lens refractive power is likely to cause an increase in the lens center thickness, making it difficult to achieve the desired thinness of the imaging lens. Thus, if a negative refractive power lens is employed, there is a problem to be addressed in order to satisfy the demand for a thin imaging lens. For example, the imaging lens described in Japanese Patent No. 4858648 (Patent Document 1) is known as an example of this type of imaging lens composed of five lenses.
Patent Document 1 discloses an imaging lens including, in order from an object side, a first lens with positive refractive power having a convex surface on the object side, a second lens with negative refractive power having a concave surface on an image side, a third lens with positive or negative refractive power, a fourth lens with positive refractive power having a convex surface on the image side, and a fifth lens with negative refractive power having a concave surface on the image side, in which the image-side surface of the fifth lens is aspheric and has an inflection point in a position other than a point of intersection with an optical axis.
The imaging lens described above in Patent Document 1 has brightness with an F-value of 2.0 or so and includes two or three negative lenses among its five lenses so that Petzval sum can be corrected easily and high imaging performance is ensured. However, since this configuration includes two or more negative lenses, it is disadvantageous in making the total track length short and has a difficulty in meeting the growing need for thinner devices. In fact, the ratio of total track length (TTL) to maximum image height (ih) (TTL/2ih) of the disclosed imaging lens is calculated as about 1.0, which suggests that it seems relatively compact and thin but its total track length is not short enough. Furthermore, since it uses a glass material, it is difficult to reduce cost. Therefore, this imaging lens cannot be said to provide the thinness, small F-value, and wide angle of view which have been expected in recent years and achieve cost reduction.